Wide bandgap device in parallel with a device that has a lower avalanche breakdown voltage and a higher forward voltage drop than the wide bandgap device

ABSTRACT

A wide bandgap device in parallel with a device having a lower avalanche breakdown voltage and a higher forward voltage drop than the wide bandgap device.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. Provisional PatentApplication Ser. No. 60/833,362, filed on Jul. 26, 2006, whichapplication is hereby incorporated by reference.

FIELD OF THE INVENTION

This invention relates to semiconductor devices, and more specifically,to a method and device for protecting wide bandgap diodes from failingduring suppression of voltage transients.

BACKGROUND OF THE INVENTION

Wide bandgap (“WBG”) semiconductors are emerging as materials capable ofdriving continued device performance enhancements for many years tocome. These materials are typically defined as exhibiting an electronicbandgap greater than 2 eV. Silicon Carbide (SiC) has been studied fordecades, but recent developments have firmly established SiC commercialproducts in optical, RF, and power components. With extremely highthermal and chemical stability, and electrical performance, WBG devicesare used for high frequency, high temperature, and high powerapplications. Examples of WBG semiconductors are gallium nitride (GaN,E_(G)=3.4 eV), aluminum nitride (AlN, E_(G)=6.2 eV), and silicon carbide(SiC, E_(G) between 2.2 to 3.25 eV depending on polytype).

Wide bandgap devices afford significant performance advantages comparedto silicon semiconductor devices. Silicon carbide Schottky diodes arefinding widespread use because of their fast switching speed (low trr),low stored charge (Qrr) and low forward conduction loss. However, widebandgap devices (e.g., silicon carbide) have poor and erratic avalanchecapability due to starting substrate quality. Avalanche breakdown is aform of electric current multiplication that can allow very largecurrents to flow within materials which are otherwise good insulators.Avalanche breakdown can occur within solids, liquids, or gases when thevoltage applied across the insulating material is great enough toaccelerate free electrons to the point that, when they strike atoms inthe material, they can knock other electrons free. This phenomenon canbe quite useful in semiconductor diodes such as the avalanche diode andavalanche photodiode, but in other situations, such as in MOSFETtransistors, avalanche breakdown can destroy the device. When avalanchebreakdown occurs within a solid insulating material it is almost alwaysdestructive. However, improvements in wide bandgap avalanche capabilityis desired.

A common avalanche diode application is protecting electronic circuitsagainst damaging high voltages. The avalanche diode is connected to thecircuit so that it is reverse-biased. In other words, its cathode ispositive with respect to its anode. In this configuration, the diode isnon-conducting and does not interfere with the circuit. If the voltageincreases beyond the design limit, the diode undergoes avalanchebreakdown, limiting the harmful voltage. When used in this fashion theyare often referred to as clamp diodes because they “clamp” the voltageto a predetermined maximum level. Avalanche diodes are normallyspecified for this role by their clamping voltage V_(BR) and the maximumsize of transient they can absorb, specified by either energy (injoules) or I²t. Avalanche breakdown is not destructive, as long as thediode is not allowed to overheat.

Paralleling a simple zener diode with the wide bandgap device can resultin the injection of minority carriers in the wide bandgap device, thusdegrading the switching performance of the wide bandgap device. Asdiscussed in U.S. Pat. Nos. 6,144,093 issued to Davis et. al. (“Davis”),and U.S. Pat. No. 5,544,038, issued to Fisher et. al. (“Fisher”), when aMOSFET or other MOS gated semiconductor device is used in switchingapplications in which the gate of the device is repeatedly turned on andoff, transient currents can flow through the body diode of the devicewhen the device is turned off, increasing the turnoff time of thedevice. One solution has been to place a Schottky diode in parallel withand oriented in the direction of the body diode to provide a faster pathfor the flow of transient currents. Typically, the Schottky diode has aforward voltage drop of about 0.4 V, whereas the body diode typicallyhas a forward voltage drop of 0.7 V. The Schottky diode thus preventsthe body diode from conducting because the Schottky diode generally hasa lower forward voltage drop than the body diode. However, unlike oneembodiment of the present invention, Davis requires at least twowirebond connections and a third connection, and Fisher requires atransistor.

Clamp protection for wideband gap devices that prevents injection ofminority carriers in the clamping device which results in degrading theswitching speed advantage is desired.

Further, clamp protection that is easy to implement monolithically andprovides uniform field distribution for good wide bandgap devicetermination edge breakdown voltage (“BV”) is desired.

Further, clamp protection that has offsetting temperature coefficientsof forward and reverse junctions to provide minimal BV temperaturecoefficients is desired.

Even further, clamp protection that allows avalanche capability to beindependently scaled is desired.

SUMMARY OF THE INVENTION

The invention comprises, in one form thereof, a wide bandgap device withimproved avalanche capability created by placing a multiple series ofdiodes across the blocking junction of a wideband gap device.

More particularly, the invention includes a monolithic combination ofback to back polysilicon diodes across a part or the entire perimeter ofthe termination of a wide bandgap diode.

In another form, the invention includes a silicon vertical PNPtransistor disposed in parallel with a wide bandgap device.

In another form, the invention includes a method for protecting widebandgap devices from failing during suppression of voltage transients.The method comprises the steps of paralleling a clamping device with awide bandgap device so that the reverse transient energy is absorbed bythe clamping device. Wherein, the clamping device maintains a BV duringavalanche less than the wide bandgap device. Further the clamping devicehas a higher forward voltage drop Vf than the wide bandgap devicepreventing forward conduction through the clamping device.

An advantage of one or more embodiments of the present invention is thatthe wide bandgap device can be rated for avalanche over the ratedoperating temperature.

A further advantage of one or more embodiments of the present inventionis that the clamping device provides a lower avalanching junction thatmaintains a lower BV than the wide bandgap device during avalanche orUIS.

A further advantage of one or more embodiments of the present inventionis to reduce the degradation of switching speed by providing a higherforward voltage drop in the clamping device that prevents the injectionof minority carriers in the clamping device during forward bias.

A further advantage of the present invention is that one or moreembodiments thereof uses a simple vertical open base transistor used inInsulated Gate Bipolar Transistors (“IGBTs”).

An even further advantage of one or more embodiments of the presentinvention is that PNP gain is optimized by controlling the peak bufferconcentration and charge, to thus minimize the temperature coefficientof collector-to-emitter breakdown (“BVceo”).

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is disclosed with reference to the accompanyingdrawings, wherein:

FIG. 1 is a cutaway side view of back to back polysilicon diode clampingplaced across the edge termination of a wide bandgap device; and

FIG. 2A is a schematic of vertical PNP BVceo clamping for a wide bandgapdevice;

FIG. 2B is a schematic diagram of the devices shown in FIG. 2A;

FIG. 3 is a graph of the current through, and the voltage across, thedevice shown in FIG. 2 and the Schottky diode by itself when theSchottky diode transitions from forward biased to reverse biased.

Corresponding reference characters indicate corresponding partsthroughout the several views. The examples set out herein illustratesseveral embodiments of the invention but should not be construed aslimiting the scope of the invention in any manner.

DETAILED DESCRIPTION

Multi-die semiconductor packages are well known in the art, and aregenerally described in U.S. Pat. Nos. 6,40,050; 6,297,55; 6,113,632; and5,814,884; all issued to Davis et. al., and which are each incorporatedherein by reference.

The method of the present invention is accomplished by paralleling aclamping device having a lower reverse breakdown voltage BV with a widebandgap device, such that reverse transient energy is absorbed by theclamping device. The clamping device must maintain a BV during avalancheless than the wide bandgap diode. To prevent conduction through theclamping device when the wide bandgap device is forward biased, theclamping device must have a higher voltage drop than the wide bandgapdevice.

Referring to FIG. 1, there is shown one embodiment 100 of a wide bandgapdevice consisting of a SiC Schottky diode 120 and a clamping device 110according to the present invention. The improvement in wide bandgapavalanche capability is achieved by placing a multiple series or “chain”of polysilicon diodes 110, consisting of alternating P doped regions 114and N doped regions 118, across the blocking junction of the Schottkydiode 120. The polysilicon diodes 110 can be placed back to back or in aseries stack, where the voltage drop of the polysilicon diodes 110 isgreater than the Schottky diode Vf. One example is a monolithiccombination of back to back polysilicon diodes 110 across part or theentirety of the perimeter of the edge termination of the Schottky diode120. The Schottky diode 120 further comprises a Schottky metal anode130. The polysilicon diodes 110 are connected between anode metal 140and cathode metal 150, and are built upon an oxide layer 160 situatedabove a junction termination extension (JTE) 170 and the silicon carbiden-type substrate 180. The polysilicon diodes are situated below adielectric region 190. In another embodiment, the polysilicon diodes area separately bonded device and are not situated across the terminationof the WBG device (not shown).

In use, this clamping is implemented monolithically across the WBGdevice edge termination and can by itself provide uniform fielddistribution for good edge BV. The advantage to this method is that thetemperature coefficient of the clamp voltage is near zero due to theoffsetting of reverse and forward junctions. However, the current levelof the unclamped inductive switching (“UIS”) or avalanche protectiondepends on the area of the polysilicon diodes, and is limited by thediodes' parasitic resistance in avalanche.

Turning to FIG. 2A, an alternative embodiment 200 of the presentinvention is shown. An open base silicon vertical PNP transistor 205 isplaced in parallel with a silicon carbide wide bandgap, high voltage(e.g., rated 600V or above) Schottky diode 210. As well known in theart, a traditional PNP transistor is formed by introducing a thin regionof N-type semiconductor material between two regions of P-type material.In this embodiment, the PNP transistor 205 has a P doped collectorregion 215, an N-doped base layer 220, an N doped buffer layer 225, anda P doped emitter layer 230. Metallization 235 contacts the collectorregion 215. The Schottky diode 210 has a Schottky junction metal layer230 which contacts metallization 236. The substrate 240 of the Schottkydiode is N doped SiC.

FIG. 2B is a schematic representation of the structure shown in FIG. 2A.Diode 250 represents the Schottky diode 210, diode 255 represents the PNjunction between the collector region 215 and the N-layer 220, and diode260 represents the PN junction between the buffer layer 225 and theemitter layer 230.

The PNP transistor 205 and Schottky diode 210 can be bond connected withseparate wires or leads 270 and 275 internal or external to the package.For example, the devices can be packaged by methods including, but notlimited to: (1) externally connecting the PNP transistor 205 andSchottky diode 210 by wire bonds; (2) mounting the PNP transistor 205and Schottky diode 210 on the same header, and connecting the topmetalizations together; or (3) connecting the PNP transistor 205 andSchottky diode 210 by a single wire bond to the lead frame or externalcircuit.

In use, the silicon PNP transistor 205 is operated in the BVceo modewhen protecting the Schottky diode 210. The buffer layer 225 causes thedevice to have asymmetrical blocking. When the circuit is forwardbiased, the PNP transistor 205 remains off and all the current flowsthrough the Schottky diode 210. When reverse biased, the PNP transistor205 breaks down before the Schottky diode 210, and all the current flowsthrough the PNP transistor 205.

The reverse junction buffer layer 225 should contain sufficient chargeto minimize BVceo snapback. The buffer layer 225 is optimized such thatthe BVceo is maintained in an acceptable range to meet the requiredminimum blocking voltage and the maximum clamp voltage over temperature.With increasing temperature during UIS, the positive temperaturecoefficient of BVceo remains lower than a PN diode because of thepositive temperature coefficient of the gain.

The thickness of the N-layer 220 should be made such that the electricfield does not punch through to the N buffer layer 225 during avalanche.

The reverse blocking voltage of the PNP transistor 205 should bedesigned high enough such that all the forward bias current flowsthrough the wide bandgap Schottky diode 210. Absent this, injectedminority carriers from the forward biased top junction and avalanchingback junction in the PNP transistor 205 will have to be removed bydepletion spread and by minority carrier recombination. This will reducethe advantage of a low Qrr and trr that can be achieved by the presentinvention. FIG. 3 illustrates a measured reverse recovery showingminimal impact on trr of the SiC Schottky diode 210 with and without thePNP transistor 205 in parallel. The Schottky diode 210 had an I_(d) of 6A, a V_(DD) of 400V, a T_(J) of 25° C., and a 500 A/μsec trr. The line420 shows the current through and the line 410 the voltage across theSchottky diode 210 when the Schotty diode 210 is in parallel with thePNP transistor 205 as show in FIG. 2 (the voltage is shown as line 410in FIG. 3, and the current is shown as line 420), and when the Schottkydiode 210 is not connected to the PNP transistor 205 (the voltage isshown as line 430, and the current is shown as line 440). As can be seenthe reverse recovery characteristics of the Schottky diode 210 areessentially unaffected by the presence of the PNP transistor 205. Thevariation shown is attributable to the capacitance of the PNP transistor205 which was about 15 picofarads for the PNP transistor 205 used togenerate the waveforms shown in FIG. 3.

It should be particularly noted that in this embodiment, PNP gain can betailored to minimize the BV_(ceo) temperature coefficient by optimizingthe depths of the N and N-layers. Additionally, only a small area die isrequired, and avalanche capability can be independently scaled byvarying the horizontal cross-sectional area of the PNP transistor 205.

While the invention has been described with reference to preferredembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof to adapt to particular situations without departingfrom the scope of the invention. Therefore, it is intended that theinvention not be limited to the particular embodiments disclosed as thebest mode contemplated for carrying out this invention, but that theinvention will include all embodiments falling within the scope andspirit of the appended claims.

1. A method for protecting a wide bandgap device during voltagetransients by adding a clamping device that has an avalanche breakdownvoltage less than said wide bandgap device and has a higher forwardvoltage drop than said wide bandgap device.
 2. The method of claim 1,wherein said clamping device is an open base PNP transistor.
 3. Themethod of claim 1, wherein said clamping device comprises one or morepolysilicon diodes in series.
 4. The method of claim 1, wherein saidwide bandgap device comprises SiC.
 5. The method of claim 1, whereinsaid wide bandgap device comprises gallium nitride.
 6. The method ofclaim 1, wherein said wide bandgap device comprises diamond.
 7. Themethod of claim 1, wherein said wide bandgap device comprises aluminumnitride.
 8. The method of claim 1, wherein said wide bandgap device is adiode.
 9. The method of claim 1, wherein said wide bandgap device is aSchottky diode.
 10. The method of claim 1, wherein said wide bandgapdevice is a MOSFET.
 11. The method of claim 1, wherein said wide bandgapdevice is a JFET.
 12. The method of claim 1, wherein said wide bandgapdevice is a bipolar transistor.
 13. The method of claim 1, wherein saidwide bandgap device is an insulated gate bipolar transistor.
 14. Thedevice of claim 2 wherein said PNP transistor is formed by introducing athin region of N-type semiconductor material between a first and asecond region of P-type material.
 15. The device of claim 2 wherein anN-type buffer layer is introduced between said N-type semiconductormaterial and said second P-type material region.
 16. The device of claim15 wherein said N-type buffer layer is sufficiently doped such that saidPNP transistor gain is low.
 17. The device of claim 15 wherein saidN-type buffer layer causes said device to have asymmetrical voltageblocking.
 18. The device of claim 16 wherein said N-type buffer layercontains sufficient charge to prevent BVceo snapback.
 19. A wide bandgapdevice and a clamping device, said clamping device having a loweravalanche breakdown voltage than said wide bandgap device and having ahigher forward voltage drop than said wide bandgap device.
 20. Thedevice of claim 19, wherein said clamping device is an open base PNPtransistor.
 21. The device of claim 20 wherein said wide bandgap devicecomprises an anode and a cathode metal.
 22. The device of claim 21wherein said open base PNP transistor is connected between said anodeand cathode metals.
 23. The device of claim 22 wherein said open basePNP transistor is wire bonded to said wide bandgap device.
 24. Thedevice of claim 19, wherein said clamping device is one or morepolysilicon diodes.
 25. The device of claim 24 wherein said wide bandgapdevice comprises an anode and a cathode metal.
 26. The device of claim25 wherein said one or more polysilicon diodes are connected betweensaid anode and cathode metals.
 27. The device of claim 26 wherein saidone or more polysilicon diodes are wire bonded to said wide bandgapdevice.
 28. The device of claim 19, wherein said wide bandgap devicecomprises silicon carbide.
 29. The device of claim 19, wherein said widebandgap device comprises gallium nitride.
 30. The device of claim 19,wherein said wide bandgap device comprises diamond.
 31. The device ofclaim 19, wherein said wide bandgap device comprises aluminum nitride.32. The device of claim 19, wherein said wide bandgap device is a diode.33. The device of claim 19, wherein said wide bandgap device is aSchottky diode.
 34. The device of claim 33 wherein said Schottky diodeis rated 600V or above.
 35. The device of claim 19, wherein said widebandgap device is a MOSFET.
 36. The device of claim 19, wherein saidwide bandgap device is a JFET.
 37. The device of claim 19, wherein saidwide bandgap device is a bipolar transistor.
 38. The device of claim 19,wherein said wide bandgap device is an insulated gate bipolartransistor.